On the ventral side of the primitive mesenteron an outgrowth appears very early, which becomes the ink sack ([fig. 127], bi).
Fig. 127. Longitudinal section through an advanced embryo of Loligo. (After Bobretzky.)
os. mouth; gls. salivary gland; brd. sheath of radula; ao. anterior aorta; ao1. posterior aorta; va. branch of posterior aorta to shell sack; ma. branch of posterior aorta to mantle; c. aortic heart; oe. œsophagus; mg. stomach; an. anus; bi. ink sack; kd. germinal tissue; eih. shell sack; vc. vena cava; g.vs. visceral ganglion; g.pd. pedal ganglion; ac. auditory sack; tr. funnel.
The mesenteric cavity, still open to the yolk, gradually extends itself in a dorsal direction over the yolk-sack, but remains for some time completely open to it ventrally, and only separated from the actual yolk by the yolk membrane. There early grow out from the walls of the mesenteron a pair of hepatic diverticula.
As the mesenteric cavity extends it dilates at its distal extremity into a chamber destined to form the stomach ([fig. 127], mg). At about this time the anus becomes perforated. Shortly afterwards the mesenteron meets and opens into the œsophagus at the dorsal extremity of the yolk-sack, but at the time when this takes place the hypoblast has extended round the entire cavity, and has shut it off from the yolk. The yolk membrane throughout the whole of this period is quite passive, and has no share in forming the walls of the alimentary tract.
The stomodæeum. The stomodæum appears as an epiblastic invagination at the anterior side of the blastoderm, before any trace of the mesenteron is present. It rapidly grows deeper, and, shortly after the mesenteric cavity becomes formed, an outgrowth arises from its wall adjoining the yolk-sack, which gives rise to the salivary glands ([figs. 126] and [127], gls). Immediately behind the opening of the salivary glands there appears on its floor a swelling which becomes the odontophore, and behind this a pocket of the stomodæal wall forms the sheath of the radula ([figs. 126] and [127], brd). Behind this again the œsophagus is continued dorsalwards as a very narrow tube, which eventually opens into the stomach ([fig. 127]).
The terminal portion of the rudiment of the salivary gland divides into two parts, each of which sends out numerous diverticula which constitute the permanent glands. The greater part of the original outgrowth remains as the unpaired duct of the two glands[117].
In the larva observed by Grenacher the anterior pair of salivary glands originated from independent lateral outgrowths of the floor of the mouth, close to the opening of the posterior salivary glands.